This invention relates to a method of transmitting user data over a synchronous digital communication network to a multiplexer for a synchronous digital communication network and to a peripheral device transmitting user data over a synchronous digital communication network.
In a synchronous digital communication network such as in SDH or SONET systems, messages are transmitted as payloads of multiplex units. Such multiplex units are referred to in SDH systems as virtual containers and in SONET as virtual tributaries. The multiplex units are transmitted in synchronous transport modules STM-N (N=1, 4, 16, 64), in which they can be positioned freely. In SDH, the largest multiplex unit is the virtual container VC-4 with a payload capacity of 149.760 Mb. Together with a pointer in the overhead of the transport module STM-1, which points to the beginning of the container, the VC-4 is called an AU-4 (administrative unit). In SONET, the largest multiplex unit is a VT-3 with a capacity of 48.384 Mb, which, together with a corresponding pointer, is called an AU-3. If payload signals whose bit rate is greater than the payload of the largest multiplex unit are to be transmitted, a concatenation of multiplex units is formed.
ITU-T G. 707, Chapter 8.1.7, describes two methods of concatenation, which can be used alternatively: “contiguous concatenation”, or “concatenation of contiguous AU-4s” (Chapter 8.1.7.1), and “virtual concatenation” (Chapter 8.1.7.2). In the contiguous concatenation method, successive multiplex units are concatenated which are transported together in a larger transport module, e.g., four AU-4s in a single STM-4, sixteen AU-4s in a single STM-16, three AU-3s in a single STM-1, or twelve AU-3s in a single STM-4. When the concatenated multiplex units are accessed, only the pointer to the first multiplex unit of the concatenation is evaluated, and this pointer value is also used for all subsequent multiplex units in the concatenation. In the virtual concatenation method, the individual multiplex units are transported independently of each other and are combined at the data sink, i.e., in the destination network element. The individual multiplex units may even be transmitted over different paths and exhibit differential delays at the data sink, which must be compensated for by buffering. Usually, only like multiplex units, i.e., multiplex units of the same size, are concatenated.
While in peripheral devices, e.g., in IP routers, which use a synchronous digital communication network as a backbone network, contiguous concatenation is preferred, because it is easier to handle, from a network point of view, virtual concatenation is preferred because of its greater flexibility. Conversion from contiguous concatenation to virtual concatenation is known per se. Each of the individual multiplex units is provided with its own pointer. An indicator in the overhead of the multiplex units indicates the concatenation. For the indicator, bits 1 and 2 of the H4 byte are currently used; the use of other overhead bytes is conceivable. The virtually concatenated multiplex units can then be transported in separate transport modules.
As mentioned, peripheral devices, such as IP routers, preferably use contiguous concatenation. In an article about the Cisco 12008 Gigabit Switch Router, retrievable from the Internet at URL “http://www.cisco.com/univercd/cc/td/doc/product/core/c isl2008/mfricg/product1.htm”, it is described that the device can be equipped with an OC-3c/STM-1c interface or an OC-12c/STM-4c interface. Both interface types use contiguous concatenation of multiplex units. Since the number of contiguously concatenated multiplex units depends on the interface type of the router, always the same number of multiplex units are transmitted regardless of the bandwidth actually used for the IP payload.